Stressful conditions help moms help their babies

Juvenile North American red squirrels that were temporarily removed from their nest so that they can be weighted. These offspring are approximately 25 days old and were quickly returned to their nest after weighing.
[Image courtesy of Ben Dantzer]

Pregnant squirrels may be able to help their babies before they are born simply by living in a crowded place, a new study in the journal Science reports.

While scientists have known that a mother can prepare her unborn baby for special conditions it may face—like lack of food—they have not understood how.

Ben Dantzer at the University of Cambridge in the UK studied red squirrels for 22 years. The populations of these animals change a lot; some years they are high and some years they are low. Here, the researchers found that when red squirrel populations were high, mothers gave birth to babies that grew at faster rates.

A fast growth rate is important; young squirrels that grow fast can more quickly become strong enough to perform daily tasks, like food gathering. This is important when the population is high, and food is scarce.

The researchers hypothesized that the fast growth rates they observed in baby squirrels in the wild were somehow caused because of the effect the high population had on their mothers. To test their hypothesis, they looked more closely at the effects of high population on squirrels by playing recordings of squirrel calls on animals in the lab. They played so many squirrel calls that the animals listening were made to think there were six times as many squirrels present than there actually were.

The researchers found that pregnant squirrels who listened to these calls had much more of a particular hormone, called glucocorticoid, in their bodies. They also observed that the babies born from these mothers grew a lot faster than pups born from mothers with normal hormone levels.

The study shows that a crowded population is a clear trigger that can cause a change in the body of the mother that ultimately helps the baby. Now scientists understand more about how mothers help their babies prepare for their unique environments before birth.